Similar question has been asked many times on mailing lists and on IRC. Sometimes it's not mail sending, but file/directory creation, or something else that generally requires some interaction with "world outside of database".
Can it be done? Sure. How, then?

I have an unusual mail setup.
My company mails are handled by gmail, and on my account there, I setup forwarding to my own mail account on my own server. Where I read the mails, and respond.
So, when someone (let's say ...